1. Technical Field
The invention relates to a silicon (Si) substrate or the like for manufacturing an electronic device, and more particularly to an electronic device that is manufactured by aligning elements formed on a front surface thereof and elements formed on a back surface thereof with high precision, a manufacturing method thereof, and a silicon substrate for the electronic device.
2. Description of the Related Art
When an electronic device is manufactured on a silicon substrate, there is a case where elements are formed on both a front surface and a back surface of the silicon substrate and the elements formed on the front surface are aligned with respect to the elements formed on the back surface. The “front surface” and “back surface” mean “one face” and “the other face” of a substrate, respectively.
For example, in a back-illuminated solid-state imaging device, as described in Japanese Patent No. 3722367 (corresponding to US 2004/005729 A), a plurality of photodiodes arranged in two-dimensional array and a signal reading-out circuit for reading out image signals detected by the photodiodes are formed on a front-surface side of the semiconductor substrate, while color filters and micro lenses are formed on a back-surface side of the semiconductor substrate.
In the back-illuminated solid-state imaging device, light is incident from a photographic subject to the back surface of the semiconductor substrate, and is received by the photodiodes provided on the front-surface side. Accordingly, when the color filter and the micro lens provided for each photodiode are not aligned with respect to the photodiode, color mixture or the like occurs.
For this reason, it is necessary to align the elements (color filers, micro lenses) provided on the back-surface side with the elements photodiodes, signal reading circuit, and the like) provided on the front-surface side.
In Japanese Patent No. 3722367, a back-surface mark is formed while viewing from the back-surface side through the semiconductor substrate, a metal layer or a silicide layer that is provided on the front-surface side. In this case, red or near-infrared light for which Si has low absorption rate is used as reference light. However, when a Si absorption layer is formed to have a sufficient thick (e.g., 10 μm) capable of absorbing most of visible light, signal intensity is hardly caught in the red reference light.
The near-infrared light having a wavelength of about 1.2 μm or less is absorbed by Si. Accordingly, the near-infrared light is not suitable for a thick Si-layer structure, and aligning with high precision cannot be achieved.
US 2006/0186560 A proposes a method for manufacturing a back-illuminated image sensor using a Si substrate in which a p layer having concentration gradient is grown by epitaxial growth on an SOI substrate with a thin seed layer deposited thereon. However, US 2006/0186560 A does not mention about forming a lattice-shaped light-shielding layer, a color filter, a micro lens, and the like on a back-surface side with high positional precision.
In the back-illuminated solid-state imaging element, when the light-shielding layer, the color filter, the micro lens, and the like provided on the back-surface side deviates from a structure of a pixel portion formed on the front-surface side of the silicon substrate, color mixture occurs. For this reason, it is important to reduce the deviation in a coordinate system between an alignment mark provided on the front-surface side and an alignment mark provided on the back-surface side.
When a size of a pixel becomes smaller than a thickness of a Si layer of a photodiode in order to arrange a large number of pixels in an imaging device, color mixture may further occur. Accordingly, a micro lens, a light-shielding element, and the like are required. As a width of the light-shielding element is increased, the color-mixture margin between two adjacent colors is also increased accordingly, but it means openings are narrowed as the color-mixture margin is increased, causing loss of sensitivity. It is noted that the color-mixture margin is the tendency of the color not to be mixed. For a micro pixel image sensor which employs a back-illuminated structure due to the decrease in pixel size and the shortage of sensitivity, there is a demand that the light-shielding element is formed as narrow as possible. Therefore, it has been desired to make the deviation in structure between the front-surface layer and the back-surface layer as narrow as possible.
The back-illuminated solid-state imaging element has been described above as an example. However, the above description is also applicable to the overall semiconductor device for forming elements aligned with high precision on the front and back surfaces.